storage: add network-dir as new storage volume type

In the 'directory' and 'netfs' storage pools, a user can see
both 'file' and 'dir' storage volume types, to know when they
can descend into a subdirectory.  But in a network-based storage
pool, such as the upcoming 'gluster' pool, we use 'network'
instead of 'file', and did not have any counterpart for a
directory until this patch.  Adding a new volume type
'network-dir' is better than reusing 'dir', because it makes
it clear that the only way to access 'network' volumes within
that container is through the network mounting (leaving 'dir'
for something accessible in the local file system).

* include/libvirt/libvirt.h.in (virStorageVolType): Expand enum.
* docs/formatstorage.html.in: Document it.
* docs/schemasa/storagevol.rng (vol): Allow new value.
* src/conf/storage_conf.c (virStorageVol): Use new value.
* src/qemu/qemu_command.c (qemuBuildVolumeString): Fix client.
* src/qemu/qemu_conf.c (qemuTranslateDiskSourcePool): Likewise.
* tools/virsh-volume.c (vshVolumeTypeToString): Likewise.
* src/storage/storage_backend_fs.c
(virStorageBackendFileSystemVolDelete): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>

include/libvirt/libvirt.h.in

@@ -2951,6 +2951,8 @@ typedef enum {
     VIR_STORAGE_VOL_BLOCK = 1,    /* Block based volumes */
     VIR_STORAGE_VOL_DIR = 2,      /* Directory-passthrough based volume */
     VIR_STORAGE_VOL_NETWORK = 3,  /* Network volumes like RBD (RADOS Block Device) */
+    VIR_STORAGE_VOL_NETDIR = 4,   /* Network accessible directory that can
+                                   * contain other network volumes */
 
 #ifdef VIR_ENUM_SENTINELS
     VIR_STORAGE_VOL_LAST